1. Technical Field
The present invention relates to a zoom lens system and a lens barrel having the same. In particular, the present invention relates to a small and high-image quality zoom lens system which is suitable for a digital still camera, a digital video camera or the like (simply also referred to as a camera, hereinafter), and a lens barrel for holding the zoom lens system.
2. Description of the Background Art
With recent progress in the development of solid-state image sensors such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal-Oxide Semiconductor) having a high pixel, digital still cameras and digital video cameras are rapidly spreading that employ an imaging device including an imaging optical system of high optical performance corresponding to a solid-state image sensor of high pixel.
Among these, especially in digital still cameras, constructions have recently been proposed in which a thin camera body is employed in order to achieve satisfactory accommodation property or portability to which the highest priority is imparted. In order to realize such digital still cameras having a thin camera body, an importance is present in taking into consideration the arrangement construction of the imaging device when not in use, that has a relatively large volume among the components of the digital still camera.
For example, Japanese Laid-Open Patent Publication No. 2003-315861 discloses a lens barrel in which in a retracted state, part of optical elements constituting an imaging optical system (typically, a lens unit in a zoom lens system) is escaped to a position different from that in an imaging state. More specifically, in the lens barrel disclosed in Japanese Laid-Open Patent Publication No. 2003-315861, in an imaging optical system composed of three lens units consisting of a first lens unit through a third lens unit, a second lens unit is escaped in a direction perpendicular to the optical axis of the first lens unit and the third lens unit, while these lens units are escaped to the image side, so that retraction is performed. As a result, the overall thickness is reduced.
Further, Japanese Laid-Open Patent Publication No. 2003-202500 discloses a construction in which in an imaging device provided with a zoom lens system, a right-angle prism provided with an internal reflecting surface for bending a light beam by 90° is arranged inside a lens unit located on the most object side. In the imaging device disclosed in Japanese Laid-Open Patent Publication No. 2003-202500, since the object light is bent into a plane perpendicular to the optical axis of the incident lens unit, the thickness of the imaging device is determined by the right-angle prism and the lens elements located on the object side relative to the right-angle prism. This reduces the thickness.
Further, Japanese Laid-Open Patent Publication No. 2004-102089 discloses a construction in which in an imaging device provided with a zoom lens system composed of four units having a construction of positive, negative, positive and positive, a right-angle prism provided with an internal reflecting surface for bending a light beam by 90° is arranged inside a second lens unit having negative optical power. In the imaging device disclosed in Japanese Laid-Open Patent Publication No. 2004-102089, the right-angle prism can be arranged inside the lens unit located on the image side relative to the first lens unit having positive optical power. This allows the right-angle prism to be constructed compactly.
Further, Japanese Laid-Open Patent Publication No. 2004-118101 discloses a zoom lens system having a retraction construction in which the lens elements are driven backward to the image side in an accommodated state so that the overall length is reduced. In the zoom lens system disclosed in Japanese Laid-Open Patent Publication No. 2004-118101, in an accommodated state, an optical filter located on the most image side in the imaging optical system is escaped in a direction perpendicular to the optical axis, so that the overall length is reduced in a retracted state.
Nevertheless, the lens barrel disclosed in Japanese Laid-Open Patent Publication No. 2003-315861 has a problem that a considerably complicated mechanism is necessary for escaping the optical element and that extremely high precision is required in machining and assembling of the components of this mechanism. Further, in the lens barrel disclosed in Japanese Laid-Open Patent Publication No. 2003-315861, since a specific optical element is solely escaped, the thickness of the imaging device is determined by the optical elements not escaped so that the thickness reduction has been limited. Furthermore, in a zoom lens system having a large magnification variation ratio exceeding 3×, four or more lens units are used in general (a multi-unit zoom lens system). Nevertheless, when the lens barrel disclosed in Japanese Laid-Open Patent Publication No. 2003-315861 is applied to such a multi-unit zoom lens system, a problem arises that the mechanism becomes complicated whereas the merit of thickness reduction became small.
Further, in the zoom lens system disclosed in Japanese Laid-Open Patent Publication No. 2003-202500, since the reflecting surface is arranged in the lens unit located on the most object side, a problem arises that a large reflecting surface is necessary. In the zoom lens system disclosed in Japanese Laid-Open Patent Publication No. 2003-202500, the thickness of the imaging device is determined by the size of the reflecting surface. Thus, it is undesirable that the reflecting surface becomes large. Further, a bright imaging optical system having a small F-number is difficult to be designed without enlarging the reflecting surface. The situation is similarly undesirable.
Further, in the zoom lens system disclosed in Japanese Laid-Open Patent Publication No. 2004-102089, the distance from the lens element located on the most object side to the reflecting surface cannot largely differ from the distance from the reflecting surface to the image sensor. This places a limit on the overall thickness reduction.
Further, in the zoom lens system disclosed in Japanese Laid-Open Patent Publication No. 2004-118101, size reduction is achieved merely for the amount corresponding to the thickness of the optical filter. This thickness reduction can be insufficient.